Optical properties of non-stoichiometric SiO2 as a function of excess silicon content and thermal treatments

The infrared (IR) absorption spectra and the behavior of the refraction index of a two-phase non-stoichiometric SiO2 film with excess Si have been studied as a function of the excess of Si and post-deposition thermal treatment. The oxides were deposited by low-pressure chemical vapor deposition using SiH4 and N2O as reactant gases at a substrate temperature in the range of 650 to 750 degrees C. Some of the films were given a final annealing treatment at temperatures ranging from 700 to 1100 degrees C in N-2 for 30 min. Both annealed and as-deposited oxides have IR absorption peaks associated with the bending, rocking and stretching modes of the Si-O-Si bonds in SiO2, although the exact location of these peaks is different for different contents of excess silicon and it also depend on the post-deposition thermal treatment given to the oxides. Unannealed samples present a shift of the stretching peak towards low wavenumbers as the excess of Si is increased. The samples annealed at 1000 degrees C on the other hand do not present this shift, Unannealed samples with large content of Si also present an absorption peak at 890 cm(-1) that could be associated with partially oxidized Si. It is suggested that at least part of the excess Si in the as-deposited samples is present in the form of an SiOx phase while in the annealed samples a clear separation occurs between a Si and a SiO2 phase. The behavior of the refraction index is similar for both types of sample, increasing as the excess silicon is increased.